Rodent models of early caregiving find that pups reared by dams providing low levels of early stimulation subsequently display heightened stress reactivity and social aggression. We examined these effects in humans by investigating the effects of early caregiving on markers of biobehavioral development at ages 2 and 3 years. This study extended the findings reported by Hane and Fox (Hane and Fox [2006] Psychol. Sci. 17: 550-556) in which 185 mothers and infants were observed and scored for variations in maternal caregiving behavior (MCB) at age 9 months. Relative to young children who received high-quality MCB in infancy, those who received low-quality MCB showed significantly higher socially inhibited behavior with adults, right frontal electroencephalographam (EEG) asymmetry, aggressive play, and maternal reported internalizing behavior problems and anger proneness. These effects were independent of early temperamental reactivity. Results parallel rodent models and demonstrate that ordinary variations in MCB influence stress reactivity and social behavior in young children.